Ni Maraging Steel Research Articles

Effects of a High Magnetic Field on Bainitic and Martensitic Transformations in Steels

Effects of magnetic fields on transformation temperature, transformation behavior and transformed structure have been investigated for bainitic transformation in an Fe-3.6Ni-1.45Cr-0.5C steel and for transformation to lath martensite in 18Ni maraging steel. Bs temperature was determined by observing the transformed structure and Ms temperature was measured from the cooling curve of the specimen. It was found that both Bs and Ms temperatures increase with increasing applied magnetic field. Bainitic transformation behavior is accelerated by the applied magnetic field. Elongated and aligned structures were observed for austenite to ferrite transformation in an Fe-0.4C alloy, but no elongation or alignment of transformed structure has been observed for transformations to bainite and lath martensite.

Structure heredity, aging and stability of strengthening of Cr–Ni maraging steels

Aging processes, structure heredity and stability of strengthening of maraging steels (including nitrogen-containing ones) subjected to thermal and thermomechanical treatment, were investigated using X-ray structure analysis, light and electron microscopy and mechanical testing. The thermomechanical treatment is accompanied by inheritance of the substructure of the deformed austenite by martensite. That results in an enhancement of strain aging and thermal stability of the martensite strengthening. The nitrogen-alloying results in austenite aging, changes its composition, and correspondingly retards recrystallization but differently influences thermal and mechanical stability of the austenite, martensitic transformations and martensite aging. Kinetics of the aging as well as cooling- and strain-induced martensitic transformations were studied. Nitrogen-containing two-phase steels having σ b = 1300–1500 MPa are recommended as economically alloyed maraging steels.

Effects of a high magnetic field on bainitic transformation in Fe-based alloys

It has been found that magnetic fields affect solid/solid phase transformation behavior and structures since helium-free superconducting magnet has been developed, and magnetic fields are new effective tools for the structural and functional control of materials. Structures of materials are controlled by magnetic field not only due to the difference of magnetic moment between parent and product phases but also magnetocrystalline anisotropy, shape magnetic anisotropy, induced magnetic anisotropy and magnetostriction. However, studies on the effects of magnetic field on bainitic transformation are very few. So the effects of magnetic field of 10 T on transformation temperature, kinetics and transformed structure for bainitic transformation have been investigated for an Fe–3.6Ni–1.5Cr–0.5C (mass%) alloy. Transformation temperature is increased by more than 40 K by applying a magnetic field of 10 T. This increment is larger than that for ferrite transformation in pure iron (9 K), pearlite transformation in Fe–0.8C (15 K) and lath martensite transformation in 18Ni maraging steel (20 K). The isothermal transformation behavior at 360 °C is accelerated by applying 10 T. The elongation or alignment of transformed structure is observed under some conditions for ferrite transformation, but has not been observed for bainitic transformation so far.

混合粉の粉末射出成形法により作製した１８Ｎｉ系マルエージング鋼の焼結と強度に及ぼすＴｉ添加量の影響

The 18Ni maraging steel is prepared by powder injection molding (PIM) process using a mixture of elemental powders with much attention to the effects of the amount of Ti addition on the flexure strength. Titanium, one of the components of the maraging steel makes it possible to precipitate the Ni3Ti particles and to accelerate the precipitation of Ni3Mo in a martensitic matrix of the steel on aging. X-ray diffraction confirms that the homogeneous alloying of all elements was completed at a relatively low temperature of 1473K, nevertheless sintering temperatures over 1563K were needed to obtain the PIM compacts with more than 95% for the relative density. The high density and strength in the PIM maraging steel is obtained at the Ti addition of 1.0 mass%.

The influence of niobium and aging treatment in the 18% Ni maraging steel

Effects of Nb (niobium) contents and aging conditions on the strength and fatigue life of 18% Ni maraging steel commonly using in aircraft, space field, nuclear energy, and vehicle, etc. were investigated. Hardness value decreased about 3% for annealed specimens and increased about 60% for 1 h aged specimens. But the values of the other specimens aged 2 h or more showed almost the same. The yield strength was the highest about 1800 MPa in 0.06% Nb specimen having twice as much as the base metal specimen. Also, the elongation was the highest in 0.03% Nb specimen showing the same as base metal specimen. The higher aging temperature and the longer aging time, the fatigue life increased. On the other hand, the 0.03% Nb specimen showed the highest fatigue life which increased about 12% more than base metal specimen. 0.06% Nb specimen aged at 482 °C for 8 h simultaneously satisfied the 250 grade strength and 200 grade elongation having the most superior mechanical properties.

Phase Transformation in Fe-Based Alloys in High Magnetic Fields

The effects of a high magnetic field on phase transformation behaviors and microstructures in Fe-based alloys have been extensively studied. It was found that a magnetic field accelerates ferritic and martensitic transformation, changes the morphology of the transformed microstructures and increases the A3 and A1 temperature. In a magnetic field of 10 Tesla, the A1 temperature increases by about 15°C for Fe-0.8C, the A3 temperature for pure Fe increases by about 8°C and the martensitic transformation temperature Ms in 18Ni maraging steel increases by 20°C. Ferrite grains are elongated and aligned along the direction of magnetic field in Fe-0.4C and Fe-0.6C alloys by ferritic transformation, but elongation was not found in pure Fe, Fe-0.05C alloy and Fe-1.5Mn-0.11C-0.1V alloy. Aligned structure was not found either by pearlitic transformation in Fe-0.8C alloy or by cementite precipitation from martensite.

Effect of Humidity on Fatigue Strength of Maraging Steel

Rotating bending fatigue tests were carried out for an 18%Ni maraging steel in order to investigate the effect of humidity on a crack initiation and its propagation of the steel. Moreover, the effect of reverted austenite on the sensitivity of fatigue strength for humidity was also investigated. Many cracks were initiated at the specimen surface and intergranular cracks were observed on the fracture surface when humidity was high. Both of the crack initiation and the early stage of the crack propagation were accelerated by humidity, especially the effect on the crack initiation was large. However, the effect of humidity on the crack initiation and its propagation was decreased and the intergranular cracking was suppressed by the formation of reverted austenite.

Behaviour of 12% Ni steel with the change of initial precipitates

Many changes have appeared in the field of die tool making in the recent years. New materials and machining techniques such as Laser Sintering or High Speed Machining enable toolmakers to increase their production and constantly raise the level of product quality. Maraging steels present the substantial group of relatively new ferrous alloys with some special features: excellent weldability, favourable mechanical properties and good heat resistance, long tool life and nevertheless user friendliness during the heat treatment process.Mechanical properties of maraging steels depend substantially on the characteristics of precipitated phases, which harden the martensitic structure during ageing. The type and shape of individual precipitates, their size and density affect the mechanical properties of these steels.The paper presents some mechanical and microstructural properties of 12% Ni maraging steel after precipitation annealing with laser-heat induced initial precipitates, predominantly the Ni3Mo, NiMn and Ni3Ti type. Considerable changes of initial precipitated phases were achieved with continuous and pulse type of laser-heat assisted precipitates initiation and growth. Technologic parameters for the initiation of precipitates may be reproduced with a common 10.6μm industrial CO2 laser, defocused to energy input of 17–300J/mm2 on each passage, a pulse duration from 10μs to continuous and a feed rate from 0.05 to 0.9m/min.

The influence of an intermediate austenitization heat treatment in the texture of cold-rolled and aged 18% Ni maraging steel

Texture analysis of cold-rolled maraging 350 steel (40%, 60%, 80%, and 90%) and the influence of an intermediate austenitization at 820 °C have been studied. Results show that the crystallographic texture increases with cold rolling. The austenitization process partially destroys the rolling texture and aging after austenitization develops crystallographic texture similar to rolling texture.

Microstructure of ausformed lath martensite in 18%Ni maraging steel

The microstructure of ausformed lath martensite in 18%Ni maraging steel was studied by analyzing electron backscatter diffraction pattern obtained by scanning electron microscopy and Kikuchi diffraction pattern obtained by transmission electron microscopy. In non-ausformed lath martensite structure, blocks and packets are clearly observed by optical microscopy. By ausforming of 60 % at 773 K, packet and block widths of lath martensite decrease whereas the packet is elongated along rolling direction. A packet of ausformed lath martensite contains some laths which belong to a crystallographically different packet. The dislocation density in ausformed lath martensite is higher than that in conventional lath martensite. It is concluded that ausforming refines the effective grain size and increases the dislocation density in lath martensite structure.

A dilatometric study of the continuous heating transformations in 18wt.% Ni maraging steel of grade 350

Dilatometric studies in 350 grade maraging steel (M350) were carried out to study the effect of heating rate and cold rolling, on precipitation, and martensite to austenite transformation. Recrystallized and cold rolled materials were subjected to controlled heating–holding–cooling cycles, under vacuum. The martensite to austenite transformation splits into two steps at lower heating rates. The first step enhanced by slow heating rate, occurs through a diffusion process, while the second step, enhanced by a fast heating rate, occurs though a shear process. The extent of the total martensite to austenite transformation is independent of heating rate, whereas the extent of precipitation decreases with increasing heating rate. Cold rolling of maraging steel results in a dilatational anomaly, which decreases with heating rate. At a given heating rate, cold rolling does not affect the extent of precipitation although it marginally lowers the precipitation start temperature and rate.

Stress corrosion cracking of maraging steel weldments

The stress corrosion characteristics of 18 wt-%Ni (MDN-250) maraging steel and its weldments made under different welding conditions have been investigated. The threshold stress intensity factor KISCC in stress corrosion conditions has been determined in 3.5 wt-%NaCl environment for the base metal and weldments. The fractured surfaces were analysed to study the types of fracture during stress corrosion cracking in base and weld metals. Fracture toughness tests were carried out and the results obtained from these tests have been compared with KISCC values.

Influence of Reversion Austenite on Initiation and Propagation of Fatigue Crack of Maraging Steel.

Rotating bending fatigue tests were carried out for 18%Ni maraging steel in order to investigate the influence of reversion austenite on crack initiation and its propagation of maraging steel. Fatigue strength was increased by formation of reversion austenite. The increase in the fatigue strength was large near the fatigue limit. This was caused from that the crack initiation and its early propagation were suppressed by the formation of reversion austenite and the suppression was marked with decrease in stress level. Although the fatigue strength decreased by humidity, the formation of reversion austenite relieved the sensitivity of fatigue strength for humidity.

716 マルエージング鋼の切欠感度に及ぼす逆変態オーステナイトの影響

Rotating bending fatigue tests were carried out for 18% Ni maraging steel in order to investigate the influence of reversion austenite on notch sensitivity of maraging steel. Fatigue strength was increased by formation of reversion austenite. The increase in the fatigue strength was large near the fatigue limit. In the range of notch radius ρ larger than 0.05 mm, fatigue limit was determined by the limit for crack initiation in the material without reversion austenite, whereas a non-propagating crack was observed in only the notched specimen of ρ=0.05 mm in the material formed reversion austenite. These were caused by suppressions of the crack initiation and its early propagation due to formation of reversion austenite. However, the influence of reversion austenite on notch sensitivity was very small.

715 マルエージング鋼の疲労き裂の発生及び伝ぱに及ぼす逆変態オーステナイトの影響

Rotating bending fatigue tests were carried out for 18%Ni maraging steel in order to investigate the influence of reversion austenite on crack initiation and its propagation of maraging steel. Fatigue strength was increased by formation of reversion austenite. The increase in the fatigue strength was large near the fatigue limit. This was caused from that the crack initiation and its early propagation were suppressed by the formation of reversion austenite and the suppression was marked with decrease in stress level. Although the fatigue strength decreased by humidity, the formation of reversion austenite relieved the sensitivity of fatigue strength for humidity.

Influence of Reversion Austenite on Notch Sensitivity of Maraging Steel.

Rotating bending fatigue tests were carried out for 18% Ni maraging steel in order to investigate the influence of reversion austenite on notch sensitivity of maraging steel. Fatigue strength was increased by formation of reversion austenite. The increase in the fatigue strength was large near the fatigue limit. In the range of notch radius ρ larger than 0.05 mm, fatigue limit was determined by the limit for crack initiation in the material without reversion austenite, whereas a non-propagating crack was observed in only the notched specimen of ρ=0.05 mm in the material formed reversion austenite. These were caused by suppressions of the crack initiation and its early propagation due to formation of reversion austenite. However, the influence of reversion austenite on notch sensitivity was very small.

Dimensional anisotropy in 18 pct Ni maraging steel

Annealing after hot rolling or extrusion produces anisotropic shape changes in 18 pct Ni maraging steel. It has been observed that the material contracts along the longitudinal direction but expands perpendicular to it. This peculiar behavior has been studied in situ by means of a dilatometer. The dimensional anisotropy (DA) was found to decrease with an increase in the annealing temperature. The metallographic investigations have led to the conclusion that DA is related to the presence of banding in the material.

Effect of Shot Peening on Fatigue Strength of 18%Ni Maraging Steel

Rotating bending fatigue tests were carried out on an 18%Ni maraging steel to investigate the effect of shot peening on fatigue strength. Fatigue strength was increased markedly by shot peening. The increase in fatigue strength due to shot peening depended on shot size and stress level. Fracture mode changed from surface fracture in the short life region to interior fracture, i.e., fish-eye fracture, in the long-life region. In the middle-life region, however, cracks initiated from both the surface and the inside of the material individually coalesced and led to the final fracture. These results were discussed on the basis of the effects of shot size on surface roughness, the depth and magnitude of compressive residual stress and work hardening.

Tensile properties and fracture toughness of 18Ni (250 grade) maraging steel weldments

Gas tungsten arc welding was performed on 18Ni (250 grade) maraging steel sheet using two different filler wires: one of the same composition as the base material and the other containing more cobalt and aluminium and less molybdenum and titanium. Weld specimens were then aged under four different sets of conditions. After metallographic characterisation, mechanical properties including hardness, tensile strength and ductility, and fracture toughness were evaluated. Results showed that use of the matching filler material led to lower strength but higher ductility than in the other case; this was attributed to the presence of reverted austenite in the former (caused by segregation, especially of molybdenum) at the fusion zone substructure boundaries. In both types of weld metal, a re-solution treatment followed by aging at 480°C resulted in optimum tensile properties. Fracture toughness of the aged weldments was in general close to that of the parent material aged at 480°C; some deterioration occurred only when welds with pronounced segregation were made at high temperature.

Microstructural changes during welding and subsequent heat treatment of 18Ni (250-grade) maraging steel

Sheet material specimens from 18Ni (250-grade) maraging steel were gas tungsten-arc welded using two different filler wires: one matching the base material in composition and the other with higher cobalt but lower molybdenum and titanium contents. Welding was carried out both in constant-current and pulsed modes. Post-weld aging was performed at three different temperatures, viz., 425, 480 and 520°C. Metallographic characterization revealed pronounced segregation, presumably of Ti and Mo, along interdendritic and intercellular boundaries in the weld metal produced with filler of matching composition. This led, during subsequent aging, to austenite reversion at temperatures much lower than in wrought (unwelded) material. Segregation and austenite reversion were not noticed when Ti and Mo contents were reduced in the filler wire, except at the highest aging temperature used. The use of pulsed welding resulted in some grain refinement, but the effect was not considerable.